Method and apparatus for controlling uplink transmissions of a wireless communication system

ABSTRACT

Techniques to partition and allocate the available system resources among cells in a communication system, and to allocate the resources in each cell to terminals for data transmission on the uplink. In one aspect, adaptive reuse schemes are provided wherein the available system resources may be dynamically and/or adaptively partitioned and allocated to the cells based on a number of factors such as the observed interference levels, loading conditions, system requirements, and so on. A reuse plan is initially defined and may be redefined to reflect changes in the system. In another aspect, the system resources may be partitioned such that each cell is allocated a set of channels having different performance levels. In yet another aspect, terminals in each cell are scheduled for data transmission (e.g., based on their priority or load requirements) and assigned channels based on their tolerance to interference and the channels&#39; performance.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation of patentapplication Ser. No. 09/848,937 entitled “Method and Apparatus forControlling Uplink Transmissions of a Wireless Communication System”filed May 3, 2001, pending, and assigned to the assignee hereof andhereby expressly incorporated by reference herein.

BACKGROUND

1. Field

The present invention relates to data communication, and morespecifically to a novel and improved method and apparatus forcontrolling uplink transmissions of a wireless communication system toincrease efficiency and improve performance.

2. Background

Wireless communication systems are widely deployed to provide varioustypes of communication such as voice, data, and so on, for a number ofusers. These systems may be based on code division multiple access(CDMA), time division multiple access (TDMA), frequency divisionmultiple access (FDMA), or some other multiple access techniques.

In a wireless communication system, communication between users isconducted through one or more base stations. A first user on oneterminal communicates with a second user on a second terminal bytransmitting data on the uplink to a base station. The base stationreceives the data and can route the data to another base station. Thedata is then transmitted on the downlink from the base station to thesecond terminal. The downlink refers to transmission from the basestation to the terminal and the uplink refers to transmission from theterminal to the base station. In many systems, the uplink and thedownlink are allocated separate frequencies.

In a wireless communication system, each transmitting source (e.g.,terminal) acts as potential interference to other transmitting sourcesin the system. To combat the interference experienced by the terminalsand base stations and to maintain the required level of performance,conventional TDMA and FDMA systems resort to reuse techniques wherebynot all frequency bands or time slots are used in each cell. Forexample, a TDMA system may employ a 7-cell reuse pattern in which thetotal operating bandwidth, W, is divided into seven equal operatingfrequency bands (i.e., B=W/7) and each cell in a 7-cell cluster isassigned to one of the frequency bands. Thus, in this system everyseventh cell reuses the same frequency band. With reuse, the co-channelinterference levels experienced in each cell are reduced relative tothat if all cells are assigned the same frequency band. However, reusepatterns of more than one cell (such as the 7-cell reuse pattern used insome conventional TDMA systems) represent inefficient use of theavailable resources since each cell is allocated and able to use only afraction of the total system resources (e.g., operating bandwidth).

CDMA systems are capable of operating with a 1-cell reuse pattern (i.e.,adjacent cells can use the same operating bandwidth). First-generationCDMA systems are primarily designed to carry voice data having a lowdata rate (e.g., 32 kbps or less). Using code division spread spectrum,the low-rate data is spread over a wide (e.g., 1.2288 MHz) bandwidth.Because of the large spreading factor, the transmitted signal can bereceived at a low or negative carrier-to-noise-plus-interference (C/I)level, despread into a coherent signal, and processed. Newer generationCDMA systems are designed to support many new applications (voice,packet data, video, and so on) and are capable of data transmission athigh data rates (e.g., over 1 Mbps). However, to achieve the high datarates, high C/I levels are required and the need to control interferencebecomes more critical.

There is therefore a need in the art for techniques to control uplinktransmissions to support data transmission at high data rates andachieve better utilization of the available resources.

SUMMARY

Aspects of the invention provide techniques to (1) partition andallocate the available system resources (e.g., the spectrum) among cellsin a communication system, and (2) allocate the resources in each cellto terminals for data transmission on the uplink. Both of these may beperformed such that greater efficiency is achieved while meeting systemrequirements.

In one aspect, adaptive reuse schemes are provided wherein the availablesystem resources may be dynamically and/or adaptively partitioned andallocated to the cells based on a number of factors such as, forexample, the observed interference levels, loading conditions, systemrequirements, and so on. A reuse plan is initially defined and each cellis allocated a fraction of the total available system resources. Theallocation may be such that each cell can simultaneously utilize a largeportion of the total available resources, if desired or necessary. Asthe system changes, the reuse plan may be redefined to reflect changesin the system. In this manner, the adaptive reuse plan may be capable ofachieving very low effective reuse factor (e.g., close to 1) whilesatisfying other system requirements.

In another aspect, the system resources may be partitioned such thateach cell is allocated a set of channels having different performancelevels. Higher performance may be achieved, for example, for lightlyshared channels and/or those associated with low transmit power levelsin adjacent cells. Conversely, lower performance may result, forexample, from low transmit power levels permitted for the channels.Channels having different performance levels may be obtained by definingdifferent back-off factors for the channels, as described below.

In yet another aspect, terminals in each cell are assigned to channelsbased on the terminals' tolerance levels to interference and thechannels' performance. For example, disadvantaged terminals requiringbetter protection from interference may be assigned to channels that areafforded more protection. In contrast, advantaged terminals withfavorable propagation conditions may be assigned to channels that aremore heavily shared and/or have the greater interference levelsassociated with their use.

The ability to dynamically and/or adaptively allocate resources to thecells and the ability for the cells to intelligently allocate resourcesto the terminals enable the system to achieve high level of efficiencyand performance not matched by systems that employ conventionalnon-adjustable, fixed reuse schemes.

The invention further provides methods, systems, and apparatus thatimplement various aspects, embodiments, and features of the invention,as described in further detail below

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a diagram of a communication system that supports a number ofusers and is capable of implementing various aspects and embodiments ofthe invention;

FIG. 2 shows cumulative distribution functions (CDFs) of the C/Iachieved for a number of fixed reuse patterns for a particularcommunication system;

FIG. 3 is a flow diagram of a specific implementation of an adaptivereuse scheme, in accordance with an embodiment of the invention;

FIG. 4 is a diagram of an embodiment of a resource partitioning andallocation for a 3-cell reuse pattern;

FIG. 5 shows a CDF of the achieved C/I for a 1-cell reuse pattern withall cells transmitting at full power;

FIG. 6 is a flow diagram of an embodiment of a scheme to schedule datatransmissions;

FIG. 7 is a flow diagram of an embodiment of a priority-based channelassignment scheme;

FIG. 8 is a flow diagram of an embodiment of a channel upgrade scheme;and

FIG. 9 is a block diagram of a base station and terminals in acommunication system, which are capable of implementing various aspectsand embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a communication system 100 that supports a numberof users and is capable of implementing various aspects and embodimentsof the invention. System 100 provides communication for a number ofcoverage areas 102 a through 102 g, each of which is serviced by acorresponding base station 104. Each base station's coverage area may bedefined, for example, as the area over which the terminals can achieve aparticular grade of service (GoS). The base station coverage areas areorganized in a manner to achieve overall coverage for a designatedgeographic area. The base station and its coverage area are oftenreferred to as a “cell”.

As shown in FIG. 1, various terminals 106 are dispersed throughout thesystem. The terminals in the coverage area may be fixed (i.e.,stationary) or mobile, and are generally served by a primary (i.e.,serving) base station. Each terminal communicates with at least one andpossibly more base stations on the downlink and uplink at any givenmoment depending on whether “soft handoff” is employed and/or whetherthe terminal is designed and operated to (concurrently or sequentially)transmit/receive multiple transmissions to/from multiple base stations.The downlink refers to transmission from the base station to theterminal, and the uplink refers to transmission from the terminal to thebase station.

In FIG. 1, base station 104 a receives data transmission from terminals106 a and 106 b on the uplink, base station 104 b receives datatransmissions from terminals 106 b, 106 c, 106 d, and 106 i, basestation 104 c receives data transmissions from terminals 106 a, 106 e,106 f, and 106 g, and so on. On the uplink, the transmission from eachcommunicating terminal represents potential interference to otherterminals in the system. The downlink transmissions are not shown inFIG. 1 for simplicity.

System 100 may be a multiple-input multiple-output (MIMO) system thatemploys multiple (N_(T)) transmit antennas and multiple (N_(R)) receiveantennas for data transmission. A MIMO channel formed by the N_(T)transmit and N_(R) receive antennas may be decomposed into N_(C)independent channels, with N_(C)≦min {N_(T), N_(R)}. Each of the N_(C)independent channels is also referred to as a spatial subchannel of theMIMO channel. The MIMO system can provide improved performance (e.g.,increased transmission capacity) if the spatial subchannels created bythe multiple transmit and receive antennas are utilized.

An example MIMO system is described in U.S. patent application Ser. No.09/532,492, entitled “HIGH EFFICIENCY, HIGH PERFORMANCE COMMUNICATIONSYSTEM EMPLOYING MULTI-CARRIER MODULATION,” filed Mar. 30, 2000,assigned to the assignee of the present invention and incorporatedherein by reference. System 100 may also be designed to implement anynumber of standards and designs for CDMA, TDMA, FDMA, and other multipleaccess schemes. The CDMA standards include the IS-95, cdma2000, W-CDMAstandards, and the TDMA standards include Global System for MobileCommunications (GSM). These standards are known in the art.

In system 100, a large number of terminals share a common systemresource, namely the total operating bandwidth. To achieve the desiredlevel of performance for a particular terminal in the system, theinterference from other transmissions needs to be reduced to anacceptable level. Also, to reliably transmit at high data rates for agiven operating bandwidth, it is necessary to operate at or above aparticular carrier-to-noise-plus-interference (C/I) level. Reduction ininterference and attainment of the required C/I are conventionallyachieved by dividing the total available resource into fractions, eachof which is then assigned to a particular cell in the system.

For example, the total operating bandwidth, W, can be divided into N_(r)equal operating frequency bands (i.e., B=W/N_(r)), and each cell canthen be assigned to one of the N_(r) frequency bands. The frequencybands are periodically reused to achieve higher spectral efficiency. Fora 7-cell reuse pattern such as that supported by FIG. 1, cell 102 a maybe assigned the first frequency band, cell 102 b may be assigned thesecond frequency band, and so on, and cell 102 g may be assigned theseventh frequency band.

A communication system is typically designed to conform to a number ofsystem requirements that may include, for example, quality of service(QoS), coverage, and performance requirements. Quality of service istypically defined as every terminal in the coverage area being able toachieve a specified minimum average bit rate a prescribed percentage ofthe time. For example, the system may be required to support anyterminal within the coverage area with a minimum average bit rate of atleast 1 Mbps for 99.99% of the time. The coverage requirement maydictate that a particular percentage (e.g., 99%) of the terminals withreceived signal levels exceeding a particular C/I threshold be able toachieve the specified grade of service. And the performance requirementsmay be defined by some particular minimum average bit rate,bit-error-rate (BER), packet-error-rate (PER), frame-error-rate (FER),or some other requirements. These requirements impact the allocation ofthe available resources and the system efficiency, as described below.

FIG. 2 shows example cumulative distribution functions (CDFs) of the C/Iachieved for terminals in a communication system based on a number ofreuse patterns obtained from simulation of terminals randomlydistributed throughout the coverage area. The horizontal axis, x,represents C/I, and the vertical axis represents the probability thatthe C/I achieved for a particular terminal is less than the value shownin the horizontal axis, i.e., P(C/I<x). As shown in FIG. 2, virtually noterminals achieve a C/I worse than 0 dB. FIG. 2 also shows that theprobability of greater C/I increases with greater reuse. Thus, theP(C/I>x) for the 7-cell reuse pattern is greater than the P(C/I>x) forthe 1-cell reuse pattern.

The C/I CDFs in FIG. 2 may be used to characterize the potentialperformance of the system. As an example, assume that a C/I of at least10 dB is required to meet a minimum instantaneous bit rate of 1 Mbps for99.99% of the time. Using a reuse factor of one (i.e., N_(r)=1, everycell reuses the same channel), the probability of not achieving therequired performance (i.e., the outage probability) is approximately12%. Similarly, cell reuse factors of three, four, and seven correspondto outage probabilities of 5.4%, 3.4%, and 1.1%, respectively. Thus inorder to achieve a 10 dB C/I for 99% of the terminals, a reuse factor ofat least seven (N_(r)≧7) is required in this example.

A number of modulation schemes may be used to modulate data prior totransmission. Such modulation schemes include M-ary phase shift keying(M-PSK), M-ary quadrature amplitude modulation (M-QAM), and others. Ingeneral, bandwidth-efficient modulation schemes such as M-QAM are ableto transmit a higher number of information bits per modulation symbol,but require high C/I to achieve the desired level of performance. Table1 lists the spectral efficiency of a number of bandwidth-efficientmodulation schemes, which is quantified by the number of informationbits transmitted per second per Hertz (bps/Hz). Table 1 also lists theassumed required C/I to achieve 1% bit error rate for these modulationschemes. TABLE 1 Required C/I Modulation Modulation (in dB) SchemeEfficiency (bps/Hz) for 1% BER BPSK 1 4.3 QPSK 2 7.3 8-PSK 3 12.6 16-QAM4 14.3 32-QAM 5 16.8 64-QAM 6 20.5

The average channel efficiency, E_(CH), of a particular reuse scheme maybe determined based on the CDF of the achievable C/I for the scheme (asshown in FIG. 2) and the achievable modulation efficiency as a functionof C/I (as shown in Table 1). If the most efficient modulation scheme isused whenever possible, then the average channel efficiency, E_(CH), maybe derived as a weighted sum of the modulation efficiencies, with theweighting being determined by the probability of achieving the requiredC/I. For example, if BPSK through 64-QAM are employed by the systemwherever possible, the average channel efficiency can be computed asfollows: $\begin{matrix}{E_{CH} = {{1 \cdot {P( {4.3 < {C/I} < 7.3} )}} + {2 \cdot {P( {7.3 < {C/I} < 12.6} )}} +}} \\{{3 \cdot {P( {12.6 < {C/I} < 12.6} )}} + {4 \cdot {P( {14.3 < {C/I} < 16.8} )}} +} \\{{5 \cdot {P( {16.8 < {C/I} < 20.3} )}} + {6 \cdot {P( {20.5 < {C/I}} )}}}\end{matrix}.$

Table 2 lists (in column 2) the average channel efficiencies for variousreuse factors (e.g., 1-cell, 3-cell, 5-cell, and 7-cell). Table 2 alsoprovides (in column 3) the average spectral (i.e., overall) efficienciesfor these reuse factors, which are derived by dividing the averagechannel efficiencies by the reuse factors. From Table 2, it can beobserved that the average channel efficiency increases as reuseincreases. However, this gain in channel efficiency with increasingreuse is more than offset by the loss in overall spectral efficiencythat results from allowing each cell to use only a fraction of the totalavailable resources for the system. Thus, the overall spectralefficiency decreases with increasing reuse. TABLE 2 Average per ChannelAverage Spectral Cell Reuse Factor Efficiency Efficiency N_(r)(bps/channel) (bps/Hz/cell) 1 4.4 4.4 3 5.18 1.73 4 5.4 1.35 7 5.75 0.82

As indicated in FIG. 2 and Table 2, the C/I for a given terminal may beimproved if the interference from terminals in neighboring cells isreduced by employing a higher reuse factor. However, in a multipleaccess system composed of many cells, maximizing the C/I for a singleterminal in one cell typically implies that the resource cannot bereused in some other cells in the system. Thus, although higher C/I andhigher throughout may be achieved for some of the terminals with higherreuse factor, the overall system throughput can decrease since thenumber of terminals allowed to transmit simultaneously using the samechannel decreases with higher reuse factor.

Conventionally, systems that require high C/I operating points employfixed reuse schemes. In these fixed-reuse systems, a “channel” madeavailable for use by a terminal in one cell may only be reused inanother cell with the same channel reuse pattern. For example, considera 3-cell reuse cluster containing cells 1, 2 and 3. In this schemedifferent channel sets are allocated to each cell in this first reusecluster. The channels in the set allocated to any one cell in a reusecluster are orthogonal to the channels in the other sets allocated tothe other cells in the cluster. This strategy reduces or eliminatesmutual interference caused by terminals within a reuse cluster. Thereuse cluster is repeated throughout the network in some prescribedfashion. So for example, a second reuse cluster of cells 4, 5 and 6would be permitted to use the same channel set as cells 1, 2 and 3,respectively. The interference to terminals in the cells in the firstreuse cluster caused by terminals in the second reuse cluster is reduceddue to the increased separation between cells using the same channelset. The increased separation implies increased path loss, and lowerinterference power. While fixed reuse schemes may be used to maximizethe percentage of terminals meeting the minimum required C/I, they aregenerally inefficient because they employ a high reuse factor.

Aspects of the invention provide techniques to (1) partition andallocate the available system resources (e.g., the spectrum) among cellsin a communication system, and (2) allocate the resources in each cellto terminals for data transmission on the uplink. Both of these may beperformed such that greater efficiency than fixed reuse schemes isachieved while meeting system requirements. Certain aspects of theinvention are based on several key observations.

First, the uplink is different from the downlink since the transmissionsfrom the terminals may be coordinated by the system for increasedefficiency. The system (e.g., cells) receives information that describescertain characteristics of the terminals in the system (e.g., their pathlosses to the serving cells). This information may then be used todetermine how to best schedule terminals for data transmission on theuplink. Coordination of the uplink data transmission allows for variousbenefits such as (1) increased uplink throughput on a system-wide basis,and (2) smaller variations in performance observed by terminals in thesystem, which implies that a more uniform quality of service (QoS) maybe delivered to the terminals.

Second, the terminals in the system typically have different tolerancelevels for interference. Disadvantaged terminals such as those near thecell borders, with poor shadowing/geometry, must transmit at higherpower to overcome their large path loss. In essence, these terminalshave small link margins, where link margin is defined as the differencebetween their peak power constraint and the transmitted power needed toachieve a desired C/I operating point at the cell site. As aconsequence, these terminals are more vulnerable to interference fromother terminals and also tend to cause greater levels of interference toterminals in nearby cells. In contrast, advantaged terminals such asthose closer to the cell site, with favorable propagation loss andshadowing, are more tolerant to interference since they have larger linkmargins. In addition, these advantaged terminals tend to contribute lessto the interference power seen by terminals in other cells.

In a typical system, a large percentage of the terminals in the systemare able to achieve a C/I that equals or exceeds a setpoint. Thesetpoint is a particular C/I required to achieve the desired level ofperformance, which may be quantified as, e.g., a particular average datarate at 1% BER or 0.01% outage probability, or some other criterion. Forthese terminals, a unity reuse pattern may be employed to achieve highefficiency for the system. Only a fraction of the terminals in thesystem are typically disadvantaged at any given time. For the fractionof terminals that achieve a C/I below the setpoint, some other reuseschemes and/or some other techniques may be employed to provide therequired performance.

In one aspect, adaptive reuse schemes are provided wherein the availablesystem resources may be dynamically and/or adaptively partitioned andallocated to the cells based on a number of factors such as, forexample, the observed loading conditions, system requirements, and soon. A reuse plan is initially defined and each cell is allocated afraction of the total available system resources. The allocation may besuch that each cell can simultaneously utilize a large portion of thetotal available resources, if desired or necessary. As the systemchanges, the reuse plan may be redefined to reflect changes in thesystem. In this manner, the adaptive reuse plan may be capable ofachieving very low effective reuse factor (e.g., close to 1) whilesatisfying other system requirements.

In another aspect, the system resources may be partitioned such thateach cell is allocated a set of channels having different performancelevels. Higher performance may be achieved, for example, for lightlyshared channels and/or those associated with low transmit power levelsin adjacent cells. Conversely, lower performance may result, forexample, from low transmit power levels permitted for the channels.Channels having different performance levels may be obtained by definingdifferent back-off factors for the channels, as described below.

In yet another aspect, terminals in each cell are assigned to channelsbased on the terminals' tolerance levels to interference and thechannels' performance. For example, disadvantaged terminals requiringbetter protection from interference may be assigned to channels that areafforded more protection. In contrast, advantaged terminals withfavorable propagation conditions may be assigned to channels that aremore heavily shared and/or have the greater interference levelsassociated with their use.

The ability to dynamically and/or adaptively allocate resources to thecells and the ability for the cells to intelligently allocate resourcesto the terminals enable the system to achieve high level of efficiencyand performance not matched by systems that employ conventionalnon-adjustable, fixed reuse schemes. The techniques described herein maybe applied to any communication systems that experience interferencesuch as, for example, wireless (e.g., cellular) communication systems,satellite communication systems, radio communication systems, and othersystems in which reuse can improve performance. In one specificimplementation, these techniques may be advantageously used to improvethe spectral efficiency of a fixed-terminal, multiple accesscommunication system designed to accommodate high data rate services.

Adaptive Reuse Schemes

The adaptive reuse schemes may be designed to exploit certaincharacteristics of the communication system to achieve high systemperformance. These system characteristics include loading effects andthe terminal's different tolerance to interference.

The loading at the cells affects the overall performance (e.g.,throughput) of the system. At low loads, the available system resourcesmay be divided into sets of “orthogonal” channels, which may then beassigned to the cells, one channel set per cell in a reuse cluster.Because the channels in each set are orthogonal to the channels in othersets, interference on these orthogonal channels is low and high C/Ivalues may be achieved. As the load increases, the number of orthogonalchannels in each set may be insufficient to meet demands, and the cellsmay be allowed to deviate from the use of only the orthogonal channels.The transmissions on non-orthogonal channels increase the averageinterference levels observed in the channels used. However, by properlycontrolling the transmission levels on non-orthogonal channels, theamount of interference may be controlled and high performance may beachieved even at higher loads.

As the load increases, the number of active terminals desiring totransmit data also increases, and the pool of terminals from which acell may select to schedule for data transmission and to assign channelsalso increases. Each terminal in the pool presents interference to otherterminals in the system, and this level may be dependent (in part) onthe particular location of the terminal to the serving cell as well asother neighbor cells. In addition, terminals with greater link marginhave a greater tolerance to other-user interference. The terminals'different interference characteristics can be exploited in schedulingterminals and assigning channels to achieve tight reuse (i.e., close tounity). In particular, as the load increases, terminals with highertolerance to interference may be assigned to channels having greaterlikelihood of receiving high interference levels.

FIG. 3 is a flow diagram of an adaptive reuse scheme in accordance withan embodiment of the invention. The development of a reuse plan and theadaptation of the plan to changing system conditions may be performedconcurrent with normal operation of the communication system.

Initially, the system is characterized, at step 310, for one or moreparameters and based on information collected for the system and whichmay be stored in a database 330. For example, the interferenceexperienced by the terminals, as observed at each cell, may bedetermined and an interference characterization may be developed, asdescribed below. The interference characterization may be performed on aper cell basis, and may involve developing a statisticalcharacterization of the interference levels such as a powerdistribution. The information used for the characterization may beupdated periodically to account for new cells and terminals, and toreflect changes in the system.

A reuse plan is then defined using the developed system characterizationand other system constraints and considerations, at step 412. The reuseplan encompasses various components such as a particular reuse factorN_(r) and a particular reuse cell layout based on the reuse factorN_(r). For example, the reuse factor may correspond to a 1-cell, 3-cell,7-cell, or 19-cell reuse pattern or cluster. The selection of the reusefactor and the design of the reuse cell layout may be achieved based onthe data developed in step 310 and any other available data. The reuseplan provides a framework for operation of the system.

Additional system parameters and/or operational conditions are thendefined, at step 314. This typically includes partitioning the totalavailable system resources into channels, with the channelscorresponding to time units, frequency bands, or some other units, asdescribed below. The number of channels, N_(c), to be employed may bedetermined based on the reuse plan defined in step 312. The availablechannels are then associated into sets and each cell is allocated arespective channel set. The sets can include overlapping channels (i.e.,a particular channel may be included in more than one set). Resourcepartition and allocation are described in further detail below.

Other parameters may also be defined in step 314 such as, for example,the scheduling interval, the operating points or setpoints of the cellsin the system, the back-off factors associated with the allocatedchannel set, the back-off factor limits, the step sizes for adjustmentsto the back-off factors, and others. The back-off factors determine thereductions in the peak transmit power levels for the channels. Theseparameters and conditions, which are described in further detail below,are akin to a set of operating rules to be followed by the cells duringnormal operation.

The system then operates in accordance with the defined reuse plan andthe cells receive transmissions from terminals scheduled for datatransmission. During the course of normal operation, the systemperformance is evaluated for the defined reuse plan, at step 316. Suchevaluation may include, for example, determining the effective pathlosses from each terminal to several nearby cells and the associatedlink margins, the throughputs, the outage probabilities, and othermeasures of performance. For example, the effective link margin for eachscheduled terminal in each channel in each cell may be computed. Basedon the computed link margins, an estimate of the average throughput ofthe system can be developed as well as the individual performance of theterminals.

Once the system performance has been evaluated, a determination is madeon the effectiveness (i.e., the performance) of the defined reuse plan,at step 318. If the system performance is not acceptable, the processreturns to step 312 and the reuse plan is redefined. The systemperformance may be unacceptable if it does not conform to a set ofsystem requirements and/or does not achieve the desired performancelevel. The redefined reuse plan may include changes to various operatingparameters, and may even include the selection of another reuse patternand/or reuse cell layout. For example, if excessive interference isencountered, the reuse pattern may be increased (e.g., from 3-cell to7-cell). Steps 312 through 318 are performed iteratively until thesystem goals are achieved (e.g., maximized throughput whilesimultaneously satisfying the minimum performance requirements for theterminals in the coverage area). Steps 312 through 318 also represent anongoing process while the system is operational.

If the system performance is acceptable (i.e., does conform to thesystem requirements), a determination is then made whether the systemhas changed, at step 320. If there are no changes, the processterminates. Otherwise, database 330 is updated, at step 324, to reflectchanges in the system, and the system is recharacterized. The steps inFIG. 3 are described in further detail below.

The process shown in FIG. 3 may be performed periodically or wheneversystem changes are detected. For example, the process may be performedas the system grows or changes, e.g., as new cells and terminals areadded and as existing cells and terminals are removed or modified. Theprocess allows the system to adapt to changes, for example, in theterminal distribution, topology, and topography.

Channels

The resource sharing among cells and terminals may be achieved usingtime division multiplexing (TDM), frequency division multiplexing (FDM),code division multiplexing (CDM), other multiplexing schemes, or anycombinations thereof. The available system resources are partitionedinto fractions using the selected multiplexing scheme(s).

For TDM-based schemes, the transmission time is partitioned into timeunits (e.g., time slots or frames), and each cell is allocated a numberof time units. For each time unit, the total operating bandwidth of thesystem can be assigned to one or more terminals by the cell allocatedwith that time unit. For FDM-based schemes, the total operatingbandwidth can be divided into frequency bands, and each cell isallocated a set of frequency bands. Each cell can then assign theallocated frequency bands to terminals within its coverage areas, andthereafter (simultaneously) receive data transmission from the terminalsvia these frequency bands. For CDM-based schemes, codes can be definedfor the system and each cell may be allocated a set of codes. Each cellcan then assign the allocated codes to terminals within its coverageareas, and thereafter (simultaneously) receive data transmission viathese codes. Furthermore, combinations of these schemes can also be usedin the partitioning process. For example, certain code channels within aCDMA system may be associated with a particular time slot or frequencychannel. Common rules governing the use of these partitioned channelsare then defined.

FIG. 4 is a diagram of an embodiment of a resource partitioning andallocation for a 3-cell reuse pattern (i.e., N_(r)=3). In this example,the system resource is divided into 12 fractions. The division can beimplemented in the time, frequency, or code domain, or a combination ofthese. Thus, the horizontal axis in FIG. 4 can represent either time offrequency, depending on whether TDM or FDM is employed. For example, the12 fractions can represent 12 time division multiplexed time slots for aTDM-based scheme or 12 frequency bands for an FDM-based scheme. Each ofthe fractions is also referred to herein as a “channel”, and eachchannel is orthogonal to the other channels.

For the 3-cell reuse pattern, the system resources may be partitioned bygrouping the available channels into three sets, and each cell in a3-cell cluster can be allocated one of the channel sets. Each channelset includes some or all of the 12 available channels, depending on theparticular reuse scheme being employed. For the embodiment shown in FIG.4, each cell is allocated an equal number of channels, with cell 1 beingallocated channels 1 through 4 for transmission at full power, cell 2being allocated channels 5 through 8, and cell 3 being allocatedchannels 9 through 12. In some other embodiments, each cell may beallocated a respective channel set that can include any number ofchannels, some of which may also be allocated to other cells.

Back-Off Factors

In an aspect, a channel structure is defined and employed by the systemsuch that as the load increases, reliable performance is achieved usingthe channels a large percentage of the time. For a particular cell, itis likely that some terminals are more immune to other-cell interferencethan some other terminals. By providing a channel structure that takesadvantage of this fact, improvement in the system throughput andperformance may be realized.

For the channel structure, each cell in a reuse cluster is allocated arespective set of channels that may then be assigned to terminals in itscoverage area. Each cell is further assigned a set of back-off factorsfor the set of allocated channels. The back-off factor for eachallocated channel indicates the maximum percentage of full transmitpower that may be used for the channel. The back-off factor may be anyvalue ranging from zero (0.0) to one (1.0), with zero indicating no datatransmission allowed on the channel and one indicating data transmissionat up to full transmit power. The back-off factors result in channelscapable of achieving different performance levels.

The back-off from full transmit power can be applied in one or moreselected channels, at one or more selected time slots, by one or moreselected cells, or any combination thereof. The back-off canadditionally or alternatively be applied to selected terminals in thecell. In an embodiment, each cell applies a back-off for each channelassigned for data transmission, with the specific value for the back-offbeing based on the operating conditions of the cell such that thedesired performance is achieved while limiting the amount ofinterference to terminals in other cells

The back-off factors for each cell can be determined based on a numberof factors. For example, the back-off factors can be determined to takeinto consideration the characteristics of the terminals, the loadingconditions at the cells, the required performance, and so on. The set ofback-off factors assigned to each cell may be unique, or may be commonamong different cells in the system. In general, the channels allocatedto each cell and the assigned back-off factors may change dynamicallyand/or adaptively based on, for example, the operating conditions (e.g.,the system load).

In one embodiment, the back-off factors for each cell are determinedbased on the distribution of the achievable C/I values for the totalensemble of (active) terminals in the cell. A non-uniform weighting ofthese terminals may be applied, for example, based on their profile, asdescribed below. This weighting may be made adaptive and/or dynamic,e.g., time-of-day dependent.

On the uplink, the C/I for a specific terminal may be determined at thecell based on, for example, a pilot signal transmitted by the terminal.The C/I for the terminal is dependent on various factors including (1)that terminal's path loss to the serving (or home) cell and (2) theother-cell interference level. In a fixed-terminal system, the path lossfor a terminal does not change appreciably and the prediction of theterminal's signal level (“C”) may be accurately made. The other-cellinterference level (i.e., a portion of “I”) depends on the path lossesfrom other interfering terminals to their serving cells as well as thepath losses from these terminals to the cell of interest. Accurateestimation of the other-cell interference levels typically requires theinstantaneous knowledge of which terminals in other cells aretransmitting and their power levels.

A number of assumptions may be made to simplify the interferencecharacterization. For example, each cell may place an upper bound on theother-cell interference levels. This may be accomplished by assumingthat one terminal in each cell is allowed to transmit on each channel,in which case the worst case other-cell interference levels may bedetermined based on the assumption that the interfering terminals willtransmit at full power. Correspondingly, the worst-case C/I for eachterminal in each cell may be estimated based on the assumption that thisterminal and other interfering terminals will be transmitting at fullpower. The C/I values for the terminals in each cell may be collectedand used to characterize an effective C/I CDF for the cell.

FIG. 5 is an example of a CDF of the C/I achieved by terminals in a cellfor a 1-cell reuse pattern with one terminal transmitting at full poweron each channel in each cell. The C/I CDF provides an indication of thepercentage of terminals in the cell that have a C/I greater than aparticular C/I value when the terminals are transmitting at full power.From FIG. 5, it can be seen that terminals within the cell havedifferent C/I characteristics. These terminals may be able to achievedifferent levels of performance or, for a particular level ofperformance, may need to transmit at different power levels. Terminalswith smaller path losses to the serving cell typically have higher C/I,which implies that they will be able to achieve higher throughput.

In an aspect, the terminals in each cell are categorized based on theirlink margins, and the back-off factors are selected based on the linkmargin categorization. Using the example C/I distribution shown in FIG.5, the population of terminals may be categorized into sets, with eachset including terminals experiencing similar other-cell interferencelevels (i.e., having C/I within a range of values). As an example, theCDF shown in FIG. 5 can be partitioned into N_(c) sets, where N_(c) isthe total number of channels allocated per cell. The sets may beselected to be equal size (i.e., the same percentage of terminals isincluded in each set), although non-equal size set partitions may alsobe defined.

Table 3 identifies the N_(c)=12 terminal sets and (column 2) tabulatesthe minimum C/I for the terminals in each of the 12 terminal sets. Sincethere are 12 terminal sets and each set is equal size, each set includesapproximately 8.3% of the terminals in the cell. The first set includesterminals having C/I of 10 dB or less, the second set includes terminalshaving C/I ranging from 10 dB to 13 dB, the third set includes terminalshaving C/I ranging from 13 dB to 15 dB, and so on, and the last setincludes terminals having C/I greater than 34.5 dB. TABLE 3 Minimum C/Is(n) Terminal Set in Range (dB) (dB) β(n) 1 <10 <−5 1.0000 2 10 −51.0000 3 13 −2 1.0000 4 15 0 1.0000 5 17 2 0.6310 6 18.5 3.5 0.4467 720.5 5.5 0.2818 8 22 7 0.1995 9 24 9 0.1259 10 26 11 0.0794 11 29.5 14.50.0355 12 >34.5 >19.5 0.0112

The cells may be designed to support a particular setpoint y (oroperating point), which is the minimum required C/I in order to operateat a desired data rate with an acceptable error rate. In typicalsystems, the setpoint is a function of the instantaneous data rateselected by the terminals, and may thus vary from terminal to terminal.As a simple example, it is assumed that a setpoint of 15 dB is requiredby all terminals in the cell.

The minimum link margin, s(n), for each set of terminals can then becomputed as:s(n)=min{C/I(n)}−γ; n=1, 2, . . . , N_(c).  Eq (1)

The minimum link margin, s(n), for each set of terminals is thedifference between the minimum C/I of the terminals in the set and thesetpoint γ. The minimum link margin s(n) represents the deviation fromthe required transmit power to the setpoint based on the assumption offull transmit power from all terminals in the system. A positive linkmargin indicates that the C/I is greater than necessary to achieve thedesired level of performance defined by the setpoint. Thus, the transmitpower of these terminals may be reduced (i.e., backed-off) by the amountproportional to their link margin and still provide the desired level ofperformance.

The back-off factors for each cell may then be derived based onknowledge of the path losses to the terminals served by the cell and thecharacterization of the other-cell interference levels. If the maximumtransmit power level is normalized as 1.0, the normalized back-offfactor for each set of terminals can be expressed as:β(n)=min(1.0, 10^(−0.1·(n))); n=1, 2, . . . , N_(c).  Eq (2)

The back-off factor associated with a particular terminal set representsthe reduction in the transmit power that can be applied to that set ofterminals while still maintaining the desired setpoint γ, and thus thedesired level of performance. The back-off in transmit power is possiblebecause these terminals enjoy better C/I. By reducing the transmit powerof a scheduled terminal by the back-off factor, the amount ofinterference to terminals in other cells can be reduced withoutimpacting the performance of this terminal.

Table 3 lists the minimum link margin s(n) (in column 3) and theback-off factor (in column 4) for each set of terminals for a setpoint γof 15 dB. As shown in Table 3, channels 1 through 4 have link margins of0 dB or less and channels 5 through 12 have progressively better linkmargins. Consequently, channels 1 through 4 are operated at full powerand channels 5 through 12 are operated at progressively reduced power.The back-off factors may be imposed on transmissions from terminals inthe associated terminal sets. For example, since the terminals in set 5have C/I of 17 dB or better and a minimum link margin s(n) of 2 dB, thenthe transmit power from these terminals may be backed off to 63.1% ofpeak transmit power.

For terminals having C/I that are below the setpoint γ, a number ofoptions may be applied. The data rate of the transmission from theseterminals may be reduced to that which can be supported by the C/I.Alternatively, the interfering terminals that cause the low C/I may berequested to (temporarily) reduce their transmit power or to stoptransmitting on the affected channels until the low C/I terminals aresatisfactorily served.

In an embodiment, once the back-off factors are determined for one cellin a reuse pattern, the back-off factors for other cells in the reusepattern can be staggered. For example, for a N_(r)=3 (i.e., 3-cell)reuse pattern that operates with 12 channels and uses an N_(s)=4 channeloffset, the back-off factors for cell 2 can be offset by fourmodulo-N_(c) and the back-off factors for cell 3 can be offset by eightmodulo-N_(c). For this reuse pattern, cell 1 applies the back-offfactors associated with channel set 1 (which includes the channels andtheir back-off factors shown in the fourth column in Table 3), cell 2applies the back-off factors associated with channel set 2 (whichincludes the channels and back-off factors shown in the fourth column inTable 3 but shifted down by four channels and wrapped around), and cell3 applies the back-off factors associated with channel set 3 (whichincludes the channels and back-off factors shown in Table 3 but shifteddown by eight channels and wrapped around). A 4-channel offset isemployed in the example, but other offsets may also be used.

Table 3 tabulates the back-off factors for cells 1 through 3 using theback-off factors shown in Table 3 and a four-channel offset. Forexample, for channel 1, cell 1 applies the back-off factor associatedwith channel 1 of set 1, cell 2 applies the back-off factor associatedwith channel 9 of set 1, and cell 3 applies the back-off associated withchannel 5 of set 1. TABLE 4 β₁(n) β₂(n) β₃(n) Channel, n Cell 1 Cell 2Cell 3 1 1.0000 0.1259 0.6310 2 1.0000 0.0794 0.4467 3 1.0000 0.03550.2818 4 1.0000 0.0112 0.1995 5 0.6310 1.0000 0.1259 6 0.4467 1.00000.0794 7 0.2818 1.0000 0.0355 8 0.1995 1.0000 0.0112 9 0.1259 0.63101.0000 10 0.0794 0.4467 1.0000 11 0.0355 0.2818 1.0000 12 0.0112 0.19951.0000

At low loads, each of the cells assigns terminals to the “better”allocated channels. For the channel allocation shown in Table 4, theterminals in cell 1 are assigned to channels 1 through 4, the terminalsin cell 2 are assigned to channels 5 through 8, and the terminals incell 3 are assigned to channels 9 through 12. When the load in each cellis four terminals or less, there is no co-channel interference from theterminals in the adjacent cells (since the 12 channels are orthogonal toone another), and each terminal should be able to achieve its setpointat the cell for uplink transmission. When the load in any of the cellsexceeds four terminals, then that cell may assign certain terminals tothose channels that are not orthogonal to those of the other cells.Since the load typically varies independently in each cell, it ispossible that the non-orthogonal channel assigned will not be occupiedby any of the adjacent cells. The probability of this event (i.e., theprobability of “non-collision”) is a function of the load in each of theadjacent cells.

The channel structure with back-off may result an increase in theeffective margin observed by all terminals in the system. The back-offfactors shown in Table 4 are initially derived based on the C/I CDFshown in FIG. 5, which is generated with the assumption that terminalsin other cells are transmitting at full power. However, when theback-off factors are applied along with a staggered channel reuse schemeas shown in Table 4, the actual C/I values achieved by the terminals ineach cell may be greater than the minimum C/I values provided in column2 of the Table 3 since the interference from the terminals in othercells is reduced by the applied back-off factors.

As an illustration, consider a case where a terminal achieved a C/I of17 dB in cell 1. Cell 1 may then assign channel 5 to this terminal. Aterminal in cell 2 is allowed to transmit at full power on this channeland a terminal in cell 3 is allowed to transmit at 12.6% of full power.The 17 dB C/I for the terminal in cell 1 was computed based on fulltransmit power and worst-case interference assessment. However, sincethe power transmitted by the terminal in cell 3 is reduced from 1.0 to0.126, the effective margin for the terminal in cell 1 will increase.The actual amount of increase in the link margin depends on the pathloss from the backed-off interfering terminal (assigned to channel 5 incell 3) to cell 1.

As a simple example, the terminals in each cell may be categorized intothree different sets having 0 dB margin, 3 dB margin, and 6 dB margin.Terminals with 0 dB margin would be allowed to transmit at full power(when scheduled), terminals with 3 dB margin would be allowed totransmit at half power, and terminals with 6 dB margin would be allowedto transmit at 25% of full power. If three channels are allocated percell, the back-off factors assigned may be 1.0 for channel 1, 0.5 forchannel 2, and 0.25 for channel three. In a 3-cell reuse pattern, thechannels may be staggered so that each cell is allocated the same threechannels but with a different set of back-off factors. Table 5 lists thestaggered channel assignment for this simple example. TABLE 5 Channel, nCell 1 Cell 2 Cell 3 1 1.00 0.25 0.50 2 0.50 1.00 0.25 3 0.25 0.50 1.00

An actual system typically does not fit the idealized system modeldescribed above. For example, non-uniform distribution of terminals,non-uniform base station placement, varied terrain and morphology, andso on, all contribute to variations in the interference levels observedin each cell. The characterization of the cells and the normalization ofperformance in the cells is typically more complicated than thatdescribed above (i.e., the C/I CDFs for the cells are not likely to beidentical). Furthermore, the terminals in each cell typically seedifferent levels of interference from the terminals in other cells.Thus, more computations may be required to normalize the effectivemargins to within a particular threshold level across the cells in thesystem.

The back-off factors derived for each cell may thus be different and maynot be modulo shifted versions of the back-off factors other cells inthe reuse cluster. Moreover, different setpoints for the cells and/orchannels may also be used to achieve a level of normalized performance,if so desired. The setpoints may also be altered to achieve non-uniformsystem performance. The effect of different C/I CDFs on the back-offfactors and the adjustment of the back-off factors to improve systemperformance are described in U.S. patent application Ser. No.09/539,157, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSIONSOF A COMMUNICATIONS SYSTEM,” filed Mar. 30, 2000, assigned to theassignee of the present application and incorporated herein byreference.

A number of different schemes may be used to determine the back-offfactors for the cells. In one scheme, a procedure to determine theback-off factors is iterated a number of times, and the back-off factorsare adjusted in each iteration such that the maximum achievable setpointfor all channels is met. In an embodiment, the worst-case other-cellinterference is assumed in determining the initial back-off factors. Inanother embodiment, other values may be used instead of the worst-caseinterference levels. For example, the average, median, or 95-percentileof the other-cell interference distribution may be used to determine theinitial back-off factors. In yet another embodiment, the interferencelevels are adaptively estimated, and the back-off factors periodicallyadjusted to reflect the estimated interference levels. The back-offfactors employed by each cell may or may not be communicated toneighboring cells.

In some embodiments, a subset of the allocated channels in a cell may beprovided with some form of “protection”. The protection may be achieved,for example, by reserving one or more channels on a periodic basis forexclusive use by terminals in the cell. The exclusivity may also bedefined to be exercisable only when required, and only to the extentrequired to satisfy disadvantaged terminals. The protected channels maybe identified to neighbor cells by various means. For example, a cellmay communicate to its neighboring cells a list of channels that areprotected. The neighbor cells may then reduce or prevent datatransmission on the protected channels by terminals in their coverageareas. Channel protection may be used to serve disadvantaged terminalsthat cannot achieve the required C/I because of excessive interferencefrom the terminals in neighbor cells. For these cases, the channelprotection may be removed once the disadvantaged terminals are served.

In some embodiments, a cell may impose “blocking” (i.e., no transmissionby terminals within its coverage areas) on certain channels if thechannel conditions deteriorate to an unacceptable level (e.g., if theFER is above a certain percentage, or the outage probability exceeds aparticular threshold value). Each cell can measure the performance ofthe channels and self-impose blocking on poor performing channels untilthere is reasonable certainty that the channel conditions have improvedand that reliable communication may be achieved.

The channel protection and blocking may be performed dynamically and/oradaptively based on, for example, the conditions of the cell.

Adjustment to the Default Back-Off Factors

In embodiments that employ power back-off, the back-off factors arecomputed and provided to the cells in the system. Thereafter, each cellapplies the back-off factors when scheduling terminals for datatransmission on the uplink and assigning channels to the terminals.

In an aspect, the initial back-off factors may be adjusted dynamicallyand/or adaptively based on, for example, changes in system loading,terminal characteristics, user demands, performance requirements, and soon. The back-off factors may be adjusted using numerous schemes, some ofwhich are described below.

In one back-off adjustment scheme, the back-off factor(s) of offendingcell(s) are reduced during the period of time a disadvantaged terminalis actively communicating. As noted above, the disadvantaged terminal inmany instances is not able to achieve the desired setpoint because ofexcessive interference from a limited number of terminals in othercells.

If the disadvantaged terminal is unable to achieve the desired setpointeven when assigned to the best available channel (a condition referredto as “soft-blocking”), terminals in other cells that cause theinterference may have their transmit power temporarily reduced such thatthe disadvantaged terminal will be able to attain the desired setpoint.As an example, if the primary interference source for a disadvantagedterminal in cell 1 is a terminal in cell 2, then the transmit power ofthe terminal in cell 2 may be backed-off by an amount necessary to allowthe disadvantaged terminal to operate at the desired setpoint (e.g., anadditional 3 dB, from β(n)=x down to β(n)=0.5x).

In the above example, if the back-off factor is applied to the terminalin cell 2, then this terminal may no longer be able to meet its setpointeither, potentially causing further reductions in the back-off factorsof other cells. Therefore, adjustments may also be made to the setpointsemployed in the specified channels of the offending cells in addition tothe back-off factors. In addition, these adjustments may be made locallyas well, so that the setpoints of both cells 1 and 2 are reduced, e.g.,to values that effectively maximize their collective throughput whilestill meeting the outage criteria of the terminals in both cells.

In another back-off adjustment scheme, the offending cell(s) may betemporarily prevented from using a particular channel so that thedisadvantaged terminal may be served. The back-off factor(s) β(n) forthe effected channel(s) may be set to 0.0 for the offending cell(s).

The primary interference for a particular terminal may be co-channelinterference from another terminal in a cell in another reuse cluster.To reduce co-channel interference, the back-off factors for theoffending cell may be modified, e.g., shifted so that the back-offfactor is not high for the channel experiencing high level ofinterference.

In another back-off adjustment scheme, one or more channels may bereserved for exclusive use by each cell in the reuse pattern. Othercells in the reuse pattern are then prevented (i.e., blocked) fromtransmitting on these channels. The number of reserved channels may bebased on the load or system requirements, and may be adjusteddynamically and/or adaptively as the operating condition changes. Also,the cells may be allocated different number of reserved channels, againdepending on the system design and conditions.

The amount of power back-off to request from other cells may be obtainedin various manners. In some implementations, each cell knows theback-off factors necessary to allow disadvantaged terminals to operateat the desired setpoint. The back-off factors may be pre-computed andsaved or may be determined from prior transmissions. When adisadvantaged terminal becomes active, the cell knows the back-offfactor(s) needed for the terminal and communicate this to the offendingcell(s).

For the embodiments in which it is desired to adjust (e.g., reduce orblock) the transmit power of the terminals in the offending cells, thecell requesting the back-off adjustment can convey to the offendingcells the desired adjustment to the back-off factors to satisfy therequirements of the disadvantaged terminals. The adjustments may also besent to other cells in the system, which may then use the information toimprove the performance of these cells. The offending cells would thenapply the requested back-off factors, based on a defined back-offadjustment scheme. Such adjustment scheme may define, for example, thetime and duration for which to apply the adjustment. If an offendingcell receives back-off requests from a number of other cells, theoffending cell typically applies the maximum of the back-off factorsthat it receives from the requesting cells.

The request (or directive) to temporarily reduce or block thetransmission in other cells may be communicated to the offending cellssuch that the disadvantage terminals can be served. The request may becommunicated dynamically to the offending cells as needed, or in anorderly manner (e.g., every few frames), or by some other methods. Forexample, each cell may send its neighbor cells a list of such requestsat the start of each transmission frame with the expectation that therequests would be applied at the next transmission frame. Other methodsfor communicating requests to other cells may be contemplated and arewithin the scope of the present invention.

The back-off adjustment may be achieved using numerous methods. In onemethod, the back-off factors are sent to the neighbor cells on a dynamicbasis and are applied shortly thereafter (e.g., the next frame). Inanother method, the back-off factors are applied at predetermined time,which is known by the affected cells.

Restoration of a back-off factor to its original value may also beachieved using numerous methods. In one method, the original back-offfactor can be restored by issuing a “restore” command to the offendingcell(s). In another method, the back-off factor is gradually restores toits original value by increasing it incrementally.

In yet another method for back-off adjustment, each cell maintains aknown step size for adjusting the back-off factor in each channel. Eachcell maintains the current value of the back-off factor employed foreach channel and a step size for increasing and decreasing the back-offfactor. Thereafter, the cell adjusts the back-off factor in accordancewith the associated step size each time it receives a request to reducetransmit power.

In an embodiment, each channel of a particular cell may be associatedwith maximum and minimum limits on the back-off factor. As an example,assume that a scheduler operating in each cell schedules on common frameboundaries, i=1, 2, 3 . . . Further, let β_(m) ^(max)(n) and β_(m)^(min)(n) be the maximum and minimum values for the back-off factor forchannel n in cell m, and let δ^(up)(n) and δ^(down)(n) represent thestep sizes for increasing and decreasing the back-off factor for channeln. The back-off adjustment at frame i in cell m for channel n can thenbe expressed as:

(a) if any neighbor cells send decrease power commands at frame i:δ_(m)(n,i)=max[β_(m) ^(min)(n),β_(m)(n,i−1)·δ^(down)(n)],

(b) otherwise:δ_(m)(n,i)=min[β_(m) ^(min)(n),β_(m)(n,i−1)·δ^(up)(n)].

The maximum and minimum back-off limits may also be adjusted as desiredor necessary. For example, the maximum and minimum limits can beadjusted based on system loading or requirements.

Dynamic adjustment of the back-off factors may be equated to dynamicadjustment of the system setpoint or the maximum permitted data rate forthe channels, based on loading, performance, or some other measures. Asthe system loading increases, the setpoint may be adjusted (i.e.,decreased) to a level that permits reliable operation in the channels.Generally, the setpoint for each channel may also be made adaptive. Thisallows the data rates associated with the channels to be set differentlyas desired or necessary. Adaptation of the setpoint in each channel maybe performed locally by each cell.

Dynamic adjustment of the back-off factors may be extended such that theback-off factors for all channels in every cell can be dynamicallyadjusted. This feature allows the system to effectively adjust the powerlevel in each of the channels so that the active terminals in thespecified channels are able to meet the desired setpoint. The powers inthe channels of adjacent cells can thus become a function of, forexample, a group of active terminals in the local cell, theirrequirements, and so on. If the mix of terminals in a cell is such thatall can achieve their setpoints in their assigned channels, then thedefault back-off factors are employed. Otherwise, additional reductionsin the back-off factors (i.e., reduced transmit power) are appliedtemporarily in the offending neighbor cells in the specified channelsand for the specified duration.

When the back-off factors are allowed to be changed dynamically, ascheduler in a particular cell may not be certain of the power beingtransmitted by the neighbor cells. This can result in an ambiguity inthe actual operating points for the terminals in the local cell.Nevertheless, adjustments to the back-off factors can still be performeddynamically, for example, by basing the adjustments on the observedperformance of the affected channel.

For example, in one implementation, the cell monitors the averageframe-erasure-rate (FER) associated with a terminal in a specificchannel. If the actual C/I is lower than the setpoint, there is a higherprobability that a frame erasure will occur, thereby resulting in aretransmission of the error frame. The cell can then (1) reduce the datarate for the terminal, (2) request the terminals in the offendingcell(s) to reduce their transmit power on this channel, or do both (1)and (2).

Parameters Used for Scheduling and Channel Assignment

The adaptive reuse schemes provide a structure for allocating resourcesto terminals requesting to transmit data on the uplink. During normalsystem operation, requests to transmit data are received from variousterminals throughout the system. The cells then schedule terminals fordata transmission and assign channels to the terminals such that highefficiency and performance are achieved.

The scheduling of terminals for data transmission and the assignment ofchannels to the terminals may be achieved using various schedulingschemes and based on a number of factors. Such factors may include (1)one or more channel metrics, (2) the priority assigned to activeterminals, and (3) criteria related to fairness. Other factors (some ofwhich are described below) may also be taken into account in schedulingterminals and assigning channels and are within the scope of theinvention.

One or more channel metrics may be used to schedule terminals and/orassign channels such that more efficient use of the system resources andimproved performance may both be achieved. Such channel metrics mayinclude metrics based on throughput, interference, outage probability,or some other measures. An example of a channel metric indicative of“goodness” is described below. However, it will be recognized that otherchannel metrics may also be formulated and are within the scope of theinvention.

The channel metrics may be based on various factors such as (1) aterminal's path loss and peak transmit power to the serving cell, (2)other-cell interference characterization, (3) the back-off factors, andpossibly other factors. In an embodiment, a channel metric, d_(m)(n,k),for active terminals may be defined as follows:d _(m)(n,k)=f{β _(m)(n)·P _(max)(k)·ζ_(m)(k)/I _(m)(n)},  Eq (3)where:

-   -   β_(m)(n) is the back-off factor associated with channel n of        cell m, with 0≦β≦1 (when β_(m)(n)=0, this is equivalent to        preventing cell m from using channel n;);    -   P_(max)(k) is the maximum transmit power for terminal k;    -   ζ_(m)(k) is the path loss from terminal k to cell m;    -   I_(m)(n) is the interference power observed by cell m on channel        n; and    -   f(x) is a function that describes the “goodness” of the argument        x, where x is proportional to the C/I.

The exact computation of the other-cell interference, I_(m)(n), requiresthe knowledge of the path losses from each interfering terminal (i.e.,those assigned to the same channel n) to its serving cell as well as tocell m under consideration. The path loss to the serving cell determinesthe amount of power to be transmitted by this interfering terminal, ifpower control is used. And the path loss to cell m determines the amountof transmit power from the interfering terminal will be received at cellm as interference. Direct computation of the other-cell interference,I_(m)(n), is typically not practical since information about theinterfering terminals is normally not available (e.g., these terminalsare being scheduled and assigned by other cells at the approximatelysame time) and the path loss characterization for these terminals istypically not accurate (e.g., likely based on averages and may notreflect fading).

The other-cell interference, I_(m)(n), may thus be estimated based onvarious schemes. In one interference estimation scheme, each cellmaintains a histogram of the received interference power for eachchannel. The total receive power, I_(o,m)(n), at cell m for channel ncomprises the power, C_(k)(n), received for the scheduled terminal k inchannel n and the interference power received from other interferingterminals in other cells (plus thermal and other background noise).Thus, the other-cell interference may be estimated as:Î _(m)(n)=I _(o,m)(n)−C _(k)(n),  Eq (4)where Î_(m)(n) is the estimated other-cell interference for cell m inchannel n. The other-cell interference, Î_(m)(n), may be estimated foreach channel and at each scheduling interval to form a distribution ofthe other-cell interference for each channel. An average value, worstcase, or some percentile of this distribution may then be used as theother-cell interference I_(m)(n) in equation (3).

Various functions f(x) may be used for the channel metric. In oneembodiment, the channel metric d_(m)(n,k) represents the outageprobability for terminal k in cell m in channel n. In anotherembodiment, the channel metric d_(m)(n,k) represents the maximum datarate that may be reliably sustained at the C/I=x. Other functions mayalso be used for the channel metric and are within the scope of theinvention.

The channel metric d_(m)(n,k) represents a “score” for terminal k incell m on channel n. The channel metric may be used to scheduleterminals for data transmission or to assign channels to terminals, orboth. In scheduling terminals and/or assigning channels, a score may becomputed for each active terminal for each channel in the cell. For eachterminal, the (up to N_(c)) scores are indicative of the expectedperformance associated with the channels available for assignment. For aparticular terminal, the channel having the “best” score may be the bestchannel to assign to the terminal. For example, if the channel metricd_(m)(n,k) represents the outage probability, then the channel with thelowest outage probability is the best channel to assign to the terminal.

The channel metric d_(m)(n,k) may be computed to a degree of confidencebased on estimates of the parameters that comprise the function f(x)(e.g., the path loss from terminal k to cell m, the interfering powerI_(m)(n) observed by cell m, and so on). The value of d_(m)(n,k) may beaveraged over a time period to improve accuracy. Fluctuations in thevalue of d_(m)(n,k) are likely to occur due to small signal fading ofboth signal and interference, changes in the location of interferencesource causing changes in the interference power, and perhaps occasionalshadow (e.g., a truck blocking the main signal path). To account for thefluctuations, channels with larger back-off factors may be selected toprovide some margins, and the data rates may also be adapted based onchanges in the operating conditions.

In an aspect, terminals may be scheduled for data transmission andassigned channels based on their priority such that higher priorityterminals are generally served before lower priority terminals.Prioritization typically results in a simpler terminal scheduling andchannel assignment process and may also be used to ensure certain levelof fairness among terminals, as described below. The terminals in eachcell may be prioritized based on a number of criteria such as, forexample, the average throughput, the delays experienced by theterminals, and so on. Some of these criteria are discussed below.

In one terminal prioritization scheme, terminals are prioritized basedon their average throughput. In this scheme, a “score” is maintained foreach active terminal to be scheduled for data transmission. A cell canmaintain the scores for the active terminals it services (i.e., for adistributed control scheme) or a central controller can maintain thescores for all active terminals (i.e., in a centralized control scheme).The active status of a terminal may be established at higher layers ofthe communication system.

In an embodiment, a score φ_(k)(i) indicative of an average throughputis maintained for each active terminal. In one implementation, the scoreφ_(k)(i) for terminal k at frame i is computed as an exponential averagethroughput, and can be expressed as:φ_(k)(i)=α₁·φ_(k)(i−1)+α₀ ·r _(k)(i)/r _(max).  Eq (5)where

-   -   φ_(k)(i)=0 for i<0,    -   r_(k)(i) is the data rate for terminal k at frame i (in unit of        bits/frame), and    -   α₀ and α₁ are time constants for the exponential averaging.        Typically, r_(k)(i) is bounded by a particular maximum        achievable data rate, r_(max), and a particular minimum data        rate (e.g., zero). A larger value for α₁ (relative to α₀)        corresponds to a longer averaging time constant. For example, if        α₀ and α₁ are both 0.5, then the current data rate r_(k)(i) is        given equal weight as the score φ_(k)(i−1) from the prior        scheduling interval. The scores φ_(k)(i) are approximately        proportional to the normalized average throughput of the        terminals.

The data rate r_(k)(i) may be a “realizable” (i.e., “potential”) datarate for terminal k based on the achieved (i.e., measured) or achievable(i.e., estimated) C/I for this terminal. The data rate for terminal kcan be expressed as:r _(k)(i)=c _(k)·log₂(1+C/I _(k)),  Eq (6)where c_(k) is a positive constant that reflects the fraction of thetheoretical capacity achieved by the coding and modulation schemeselected for terminal k. The data rate r_(k)(i) may also be the actualdata rate to be assigned in the current scheduling period, or some otherquantifiable data rates. The use of the realizable data rate introducesa “shuffling” effect during the channel assignment process, which mayimprove the performance of some disadvantaged terminals, as describedbelow.

In another implementation, the score φ_(k)(i) for terminal k at frame iis computed as a linear average throughput achieved over some timeinterval, and can be expressed as: $\begin{matrix}{{\phi_{k}(i)} = {\frac{1}{K}{\sum\limits_{j = {i - K + 1}}^{i}{{r_{k}(j)}/{r_{\max}.}}}}} & {{Eq}\quad(7)}\end{matrix}$The average (realizable or actual) throughput of the terminal can becomputed over a particular number of frames (e.g., over the latest 10frames) and used as the score. Other formulations for the score φ_(k)(i)for active terminals can be contemplated and are within the scope of thepresent invention.

In an embodiment, when a terminal desires to transmit data (i.e.,becomes active), it is added to a list and its score is initialized(e.g., to zero or a normalized data rate that the terminal can achievebased on the current C/I). The score for each active terminal in thelist is subsequently updated in each frame. Whenever an active terminalis not scheduled for transmission in a frame, its data rate is set tozero (i.e., r_(k)(i)=0) and its score is updated accordingly. If a frameis received in error by a terminal, the effective data rate for thatframe is also set to zero. The frame error may not be known immediately(e.g., due to round trip delay of an acknowledgment/negativeacknowledgment (Ack/Nak) scheme used for the data transmission) but thescore can be adjusted accordingly once this information is available.

A scheduler can then use the scores to prioritize terminals forscheduling and/or channel assignment. In a specific embodiment, the setof active terminals is prioritized such that the terminal with thelowest score is assigned the highest priority, and the terminal with thehighest score is assigned the lowest priority. The scheduling processormay also assign non-uniform weighting factors to the terminal scores inperforming the prioritization. Such non-uniform weighting factors cantake into account others factors (such as those described below) to beconsidered in determining terminal priorities.

In certain embodiments (e.g., if the realizable data rate is used), thescore φ_(k)(i) for a particular terminal is not necessarily indicativeof what is supportable by the terminal (i.e., may not reflect theterminal's potential data rate). For example, two terminals may beassigned the same data rate, even though one terminal may be capable ofsupporting a higher data rate than the other. In this case, the terminalwith the higher potential data rate can be given a higher score and thuswill have a lower priority.

The priority of a terminal may also be made a function various otherfactors. These factors may include, for example, payload requirements,the achievable C/I and the required setpoint, the delays experienced bythe terminals, outage probability, interference to adjacent cells,interference from other cells, data rates, the maximum transmit powers,the type of data to be transmitted, the type of data services beingoffered, and so on. The above is not an exhaustive list. Other factorsmay also be contemplated and are within the scope of the invention.

A terminal's payload may be used to determine priority. A large payloadtypically requires a high data rate that may be supported by a smallernumber of the available channels. In contrast, a small payload cantypically be supported by more of the available channels. The smallpayload may be assigned to a channel having a large back-off factor thatmay not be able to support a high data rate needed for a large payload.Since it is more difficult to schedule data transmission for a largepayload, a terminal with the large payload can be assigned a higherpriority. In this way, the terminal with the large payload may be ableto enjoy comparable level of performance as a terminal with a smallpayload.

A terminal's achieved C/I may also be used to determine priority. Aterminal having a lower achieved C/I can only support a lower data rate.If the available resources are used for transmission to a terminalhaving a higher achieved C/I, the average system throughput would likelyincrease, thereby improving the efficiency of the system. Generally, itis more preferable to transmit to terminals having higher achieved C/I.

The amount of delay already experienced by a terminal may also beconsidered in determining priority. If resource allocation is achievedbased on priority, a low priority terminal is more likely to experiencelonger delays. To ensure a minimum level of service, the priority of theterminal can be upgraded as the amount of delay experienced by theterminal increases. The upgrade prevents a low priority terminal frombeing delayed for an intolerable amount of time or possiblyindefinitely.

The type of data to be transmitted by a terminal may also be consideredin determining priority. Some data types are time sensitive and requirequick attention. Other data types can tolerate longer delay intransmission. Higher priority may be assigned to data that is timecritical. As an example, data being retransmitted may be given higherpriority than data transmitted for the first time. The retransmitteddata typically corresponds to data previously transmitted and receivedin error. Since other signal processing at the cell may be dependent onthe data received in error, the retransmitted data may be given higherpriority.

The type of data services being provided may be considered in assigningterminal priority. Higher priority may be assigned to premium services(e.g., those charged higher prices). A pricing structure may beestablished for different data transmission services. Through thepricing structure, the terminal can determine, individually, thepriority and the type of service the terminal can expect to enjoy.

The factors described above and other factors may be weighted andcombined to derive the priorities of the terminals. Different weightingschemes may be used depending on the set of system goals beingoptimized. As an example, to optimize the average throughput of thecell, greater weight may be given to the terminals' achievable C/I.Other weighting schemes may also be used and are within the scope of theinvention.

A fairness criterion may be imposed in scheduling terminals andassigning channels to ensure (or maybe even guarantee) a minimum gradeof service (GOS). The fairness criterion is typically applied to allterminals in the system, although a particular subset of the terminals(e.g., premium terminals) may also be selected for application of thefairness criterion. Fairness may be achieved with the use of priority.For example, a terminal may be moved up in priority each time it is notscheduled for data transmission and/or with each unsuccessfultransmission.

For the terminal prioritization scheme described above, the allocationof resources may be made on the basis of the ratio of scores. In thiscase, the scores of all active terminals may be referenced to themaximum of the terminal scores to form a modified score {circumflex over(φ)}_(n)(k), which can be expressed as:{circumflex over (φ)}_(k)(i)=φ_(k)(i)/max_(k){φ_(k)(i)}.  Eq (8)

The resources allocated to a particular terminal may then be based ontheir modified score. For example, if terminal 1 has a score that istwice that of terminal 2, then the scheduling processor can allocate achannel (or a number of channels) having the capacity necessary toequalize the data rates of these two terminals (provided that suchchannel(s) are available). As a fairness consideration, the schedulercan attempt to normalize data rates at each scheduling interval. Otherfairness criteria may also be imposed and are within the scope of theinvention.

Scheduling of Data Transmissions

The cells in the system operate using an adaptive reuse plan formulatedin the manner described above and in accordance with the prescribedrules and conditions. During normal operation, each cell receivesrequests from a number of terminals in the cell for data transmission.The cells then schedule terminals for data transmission to meet thesystem goals. The scheduling can be performed at each cell (i.e., for adistributed scheduling scheme), by a central scheduler (i.e., for acentralized scheduling scheme), or by a hybrid scheme in which some ofthe cells schedule their own transmissions and a central schedulerschedules transmissions for a set of cells.

FIG. 6 is a flow diagram of an embodiment of a priority-based schedulingscheme to schedule terminals for data transmission. In thispriority-based scheduling scheme, active terminals are scheduled fortransmission based on their priority, one terminal at a time, from thehighest priority to lowest priority. The number of terminals that may bescheduled for data transmission at each scheduling interval is limitedby the number of available channels. For example, up to N_(c) terminalsper cell may be scheduled for transmission on the N_(c) availablechannels.

Initially, parameters to be used for scheduling terminals are updated,at step 610. These parameters may include the back-off factors, theother-cell interference characterization, the path losses for theterminals, and possibly others. The parameters may be used to determinethe channel metrics for the terminals.

The terminals are then prioritized and ranked, at step 612. Generally,only active terminals having data to transmit are considered forscheduling, and these terminals are prioritized and ranked.Prioritization of terminals may be performed using any one of a numberof terminal-rating schemes and may be based on one or more criterialisted above such as the average throughput, payload, and so on. Theactive terminals are then ranked accordingly based on their priorities,from highest priority to lowest priority.

The available channels are then assigned to the active terminals, atstep 614. The channel assignment typically involves a number of steps.First, one or more channel metrics are computed for each terminal foreach available channel based on the updated parameters. Any number ofchannel metrics may be used, such as the one shown in equation (3). Theterminals are then assigned to the available channels based on theirpriority, the computed channel metrics, and possibly other factors suchas demand requirements. The channel assignment may be performed based onvarious channel assignment schemes, some of which are described below.

A channel assignment can imply a channel assigned as well as a data rateto be used. Each of the possible data rates may be associated with arespective coding and modulation scheme. Each scheduled terminal mayknow (e.g., a priori) the proper coding and modulation scheme to be usedbased on the assigned data rate. Alternatively, the coding andmodulation scheme may be conveyed to the scheduled terminal. This“adaptive” coding and modulation may be used to provide improvedperformance.

System parameters are then updated to reflect the channel assignments,at step 616. The system parameters to be updated may include, forexample, adjustments to the back-off factors for the channels in thecell based on (1) the channel assignments for the scheduled terminals inthis cell, (2) requests for adjustment of back-off factors from othercells, and so on. The cell may also request adjustments of the back-offfactors by neighbor cells.

The cell then receives data transmissions from the scheduled terminalsvia the assigned channels, at step 618. From the received transmissions,the cell estimates various quantities that may be used for a futurescheduling interval, such as the interference for each channel.Generally, steps 610 through 618 are performed during normal operationof the cell. At step 620, a determination is made whether anotherscheduling interval has occurred. If the answer is yes, the processreturns to step 610 and the terminals are scheduled for the nextscheduling interval. Otherwise, the process waits at step 620. Some ofthese steps are described in further detail below.

Channel Assignment Schemes

The available channels may be assigned to active terminals based onvarious schemes and taking into account various factors. These channelassignment schemes can range in complexity and in the optimality (i.e.,quality) of the assignment results. A few channel assignment schemes aredescribed below for illustration, and these include (1) a priority-basedchannel assignment scheme, (2) a demand-based channel assignment scheme,and (3) a channel assignment with upgrade scheme. Other schemes can alsobe implemented and are within the scope of the invention.

In a priority-based channel assignment scheme, channel assignment isperformed for one terminal at a time, with the highest priority terminalbeing considered first for channel assignment and the lowest priorityterminal being considered last for channel assignment. All activeterminals in the cell are initially prioritized based on a number offactors such as those described above.

FIG. 7 is a flow diagram of an embodiment of a priority-based channelassignment scheme. Initially, channel metrics are computed for theactive terminals and for the available channels, at step 710. Variouschannel metrics may be used, such at those described above. The activeterminals are then prioritized and ranked based on the factors describedabove, at step 712. The prioritization may also be based on the computedmetrics computed in step 710. The terminal priority and channel metricsare then used to perform channel assignment.

At step 714, the highest priority terminal is selected from the list ofactive terminals, and is assigned an available channel, at step 716. Inone embodiment, the selected terminal is given the first choice ofchannel and is assigned an available channel with the best channelmetric. In another embodiment, the selected terminal is assigned anavailable channel with the worst metric that still meets the terminal'srequirements. The selected terminal is also assigned a particular datarate determined based on (1) the maximum rate required by the terminal,(2) the terminal's available transmit power and the back-off factorassociated with the assigned channel, and (3) the terminal'srequirements (e.g., outage criterion), at step 718.

The assigned terminal is then removed from the list of active terminals,at step 720. A determination is then made whether the active terminallist is empty, indicating that all active terminals have been assignedchannels, at step 722. If the list is not empty, the process returns tostep 714 and the highest priority, unassigned terminal in the list isselected for channel assignment. Otherwise, if all terminals have beenassigned channels, the process terminates.

In an embodiment, if there is a tie during the channel assignment (i.e.,if more than one channels are associated with the same or similarchannel metrics), the channels are not assigned immediately. Instead,those channels that resulted in the tie are tagged and the evaluation ofother lower priority terminals continues. If the next terminal has itslargest metric associated with any one of the tagged channels, then thatchannel may be assigned to that terminal and removed from the list ofavailable channels. When the list of tagged channels for a particularterminal is reduced to one, the remaining channel is assigned to thehighest priority terminal that tagged that channel.

If the channel assignments result in a terminal having additional linkmargin over that required for the assigned data rate (i.e., the C/I ofthe terminal on the assigned channel is greater than the setpoint), then(1) the data rate of the terminal may be increased to a level thatsatisfies the required level of performance, or (2) the transmit powerof the terminal may be reduced (e.g., by lowering the back-off factor)by up to the amount of the link margin to reduce interference in thesystem. The increased data rate of the terminal, as supported by theeffective link margin, increases the throughput for the terminal as wellas the system. Power control is thus effectively exercised for thescheduled terminal. The adjustment in data rate and/or back-off factormay be made for each scheduled terminal based on its channel assignment.

If a terminal is assigned a channel not capable of supporting thedesired data rate, several options may be applied. In one option, theterminal is scheduled to transmit at a reduced data rate (a conditionreferred to herein as “dimming”). In another option, the terminal is notpermitted to transmit in the current scheduling interval (a conditionreferred to herein as “blanking”), and the channel is made available toanother active terminal. In either case, the priority of a terminal thatis dimmed or blanked may be increased, improving the terminal's chancesfor earlier consideration in the next scheduling interval.

If the priority of a terminal is updated according to its averagethroughput, then the channel assignment scheme may consider theterminal's achievable data rate when assigning channel. In oneembodiment, the particular channel assigned to a terminal is the onethat maximizes the terminal's throughput at a given outage level. Thechannel assignment scheme can first evaluate the best channel for theterminal from the list of available channels. The maximum data rate thatsatisfies the required outage criteria is then assigned to the terminalfor that channel.

In a demand-based channel assignment scheme, the demand or payloadrequirements of the terminals are considered when making channelassignments such that the available system resources may be betterutilized. For a particular set of available channels, a terminal havinglower payload requirements (which may be satisfied with a lower datarate) may be serviced by a number of available channels whereas aterminal having higher payload requirements (which may require a higherdata rate) may be serviced by a reduced number of available channels. Ifthe terminal with the lower payload requirements has higher priority andis assigned the best available channel (among many channels that alsofulfill the terminal's requirements), and if that channel is the onlyone that can fulfill the requirements of the terminal with the higherpayload, then only one terminal will be served and the resources are noteffectively used.

As an example, consider a situation where three channels are availablefor assignment to two terminals and that terminal 1 has a payloadrequirement of 1 Kbyte and terminal 2 has a payload requirement of 10Kbytes. Further, assume that only one of the three channels will satisfythe requirement of terminal 2 whereas all three channels will satisfythe requirement of terminal 1. The channels may be assigned as follows:

-   -   (a) If terminal 2 has higher priority than terminal 1, terminal        2 is assigned the channel that maximizes its throughput.        Terminal 1 is then assigned the next best channel by default.        Both terminals are served by their channel assignments.    -   (b) If terminal 1 has higher priority than terminal 2, and if        the payload requirements of the terminals are not considered in        making the channel assignment, terminal 1 may be assigned the        channel that has the largest effective margin even though any        one of the available channels would have satisfied terminal 1's        requirement. Terminal 2 would be assigned the next best channel        by default, which may not satisfy its requirement. Terminal 2        would then be served at a lower data rate or remain in the queue        until the next scheduling period.

Several assignment options are available for case (b). If the channelassignment is performed as described above, the power used in thechannel assigned to terminal 1 can be reduced to the level required forreliable communications at the desired data rate. Another assignmentoption in case (b) is to assign terminal 1 the channel having the lowestmargin that satisfies the requirements of terminal 1. With this channelassignment, other better channels are made available for other terminalsthat may need them (e.g., because of higher payload requirements orlower achieved C/I). Using this demand or payload-based channelassignment, channels with larger margins are available for assignment tosubsequent terminals that may require the additional margins.Payload-based channel assignment may thus maximize the effectivethroughput in a scheduling interval.

A flow diagram for the demand-based channel assignment scheme may beimplemented similar to that shown for the priority-based channelassignment scheme in FIG. 7. In one embodiment, each terminal selectedfor channel assignment is assigned an available channel with the worstmetric that still meets the terminal's requirements. In anotherembodiment, the priorities of the terminals may be modified such thatterminals with larger payloads are considered for assignment earlier.Numerous other variations are also possible and are within the scope ofthe invention.

In a channel assignment with upgrade scheme, the active terminals areinitially assigned channels (e.g., based on their priorities or demandsas described above) and thereafter upgraded to better channels if anyare available. In certain embodiments of the schemes described above,higher priority terminals may be initially assigned to the worstchannels that still satisfy their requirements, and better channels aresaved for lower priority terminals in case they are needed. Theseschemes may result in successively lower priority terminals beingassigned to successively better channels associated with back-offfactors that are closer to unity (i.e., greater transmit power).

If the number of active terminals is less than the number of availablechannels, it is possible to upgrade the terminals. A terminal may beupgraded to another unassigned channel that has a higher margin than theinitial assigned channel. The reason for upgrading the terminal is toincrease reliability and/or lower the effective transmit power requiredto support the transmission. That is, since a number of unassignedchannels satisfies the terminal's requirements, reassigning the terminalto the channel with higher margin allows for reduction in the transmitpower by the amount of margin.

Various schemes may be used to upgrade channels, some of which aredescribed below. Other channel upgrade schemes may also be implementedand are within the scope of the invention.

In one channel upgrade scheme, terminals are reassigned to betteravailable channels, if these channels meet the requirements of theterminals and can provide larger link margins. The channel upgrade maybe performed based on priority such that higher priority terminal areupgraded first and lower priority terminals are upgraded later ifchannels are available. This upgrade scheme allows some or all of theactive terminals to enjoy better channels having higher link margins.

FIG. 8 is a flow diagram of an embodiment of a channel upgrade schemewhereby terminals are upgraded based on their priorities. Prior tocommencing the upgrade process shown in FIG. 8, the active terminals areassigned to their initial channel assignments, which can be achievedusing the channel assignment scheme described above in FIG. 7. At step810, a determination is made whether all available channels have beenassigned to active terminals. If all channels have been assigned, nochannels are available for upgrade and the process proceeds to step 828.Otherwise, the terminals are upgraded to the available channels, ifthese channels are better (i.e., associated with better channel metrics)than the original assigned channels.

At step 812, the highest priority terminal from the list of activeterminals is selected for possible channel upgrade. For the selectedterminal, the “best” channel from the list of unassigned channels isselected. The best channel may correspond to the channel having the“best” channel metric for the selected terminal.

A determination is then made whether an upgrade is possible for theselected terminal, at step 816. If the channel metric of the bestavailable channel is worse than that of the channel originally assignedto the selected terminal, then no upgrade is performed and the processproceeds to step 824. Otherwise, the selected terminal is upgraded tothe best available channel, at step 818, which is then removed from thelist of available channels, at step 820. The channel initially assignedto the selected terminal may be placed back on the list of availablechannels for possible assignment to some other lower priority terminal,at step 822. The selected terminal is then removed from the list ofactive terminals, at step 824, regardless of whether a channel upgradewas performed or not.

At step 826, a determination is made whether the list of activeterminals is empty. If the terminal list is not empty, the processreturns to step 810 and the highest priority in the list is selected forpossible channel upgrade. Otherwise, if no channels are available forupgrade or if all active terminals have been considered, the processproceeds to step 828 and the back-off factors for all channels areadjusted to reduce the transmit powers of the scheduled and assignedterminals. The process then terminates.

The upgrade process in FIG. 8 effectively upgrades active terminals tothe available channels that are more likely to provide improvedperformance. The channel upgrade scheme shown in FIG. 8 may be modifiedto provide improved channel upgrades. For example, for a particularterminal, it may be possible that a channel freed up by a lower priorityterminal is better for this terminal. However, the terminal is notassigned to this channel because it has already been removed from theterminal list by the time the lower priority terminal is considered. Theprocess in FIG. 8 may thus be iterated a number of times, or other testsmay be performed to account for this situation.

In another channel upgrade scheme, the assigned terminals are upgradedby the number of available channels. For example, if three channels areavailable, each scheduled and assigned terminals move up by three slots.This upgrade scheme allows most (if not all) terminals to enjoy betterchannels. For example, if channels 1 through 12 having progressivelyworse performance are available for assignments and nine terminals areinitially assigned to channels 4 through 12, then each terminal may beupgraded by three channels. The nine terminals then occupy channels 1through 9 and channels 10 through 12 may be disabled.

In another channel assignment scheme, the differences between thechannel metrics associated with the channels may be taken into accountin the channel assignment. In some instances, it may be better to notassign the highest priority terminal the channel with the best channelmetric. For example, a number of channels may be associated withapproximately similar metrics for a particular terminal, or a number ofchannels may provide the required C/I. In these instances, the terminalmay be assigned one of several channels and still be properly served. Ifa lower priority terminal has as its best channel the same one selectedby a higher priority terminal, and if there is a large disparity betweenthe lower priority terminal's best and second best channels, then it maybe more optimal to assign the higher priority terminal its second bestchannel and assign the lower priority terminal its best channel. Forexample, if terminal 1 has similar channel metrics for channels 2 and 3and the next lower priority terminal 2 has a much larger channel metricfor channel 3 than channel 2, then terminal 1 may be assigned channel 2and terminal 2 may be assigned channel 3.

In yet another channel assignment scheme, the highest priority terminaltags the available channels that provide the required performance(similar to the tagging of tied channels described above). The nextlower priority terminal then tags its acceptable channels. The channelassignment is then performed such that lower priority terminals areassigned channels first but channels needed by higher priority terminalsare reserved.

In yet another channel assignment scheme, the channels are moreoptimally assigned to active terminals in the cell by considering alarge number of permutations of channel assignments over the group ofactive terminals in the cell. In this case, the channel assignmentdecision for a particular terminal is not made on the basis of theterminal's metrics and priority alone. In an implementation, theterminal's priority can be converted into a weight that is used to scalethe metrics in the computation of the channel assignments in the cell.

Active terminals may be scheduled for transmission and assigned channelsbased on their priorities, demand, scores (e.g., as computed in equation(3)), and so on, as described above. Some other considerations forscheduling terminals for data transmission and assigning channels aredescribed below.

First, a particular terminal may be assigned to multiple channels ifsuch channels are available and if one channel is not capable of meetingthe terminal's requirements. For example, a terminal may be assigned afirst channel capable of supporting 50% of the terminal's requirements,a second channel capable of supporting 35% of the terminal'srequirements, and a third channel capable of supporting the remaining15% of the terminal's requirements. If this particular allocation ofresources prevents other terminals from achieving their requirements,then the priorities of the underserved terminals may improve such thatthey will be considered earlier for the allocation of resources insubsequent scheduling intervals.

Second, a particular terminal may be assigned to different channels fordifferent scheduling intervals to provide a “shuffling” effect. Thisshuffling of assigned channels may provide interference averaging incertain instances, which may improve the performance of a disadvantagedterminal.

Third, the probabilities of other terminals transmitting on a particularchannel can be taken into account. If a number of channels have nearlyequal channel metrics without taking into account the occupancyprobabilities, then the better channel to assign is the one that has thelowest probability of being used. Thus, the probability of channeloccupancy may be used to determine the best channel assignment.

Fourth, excessive outage probability may be considered in making thechannel assignments. In some instances, it is possible that assignmentof a channel to a particular terminal is unwarranted or unwise. Forexample, if a terminal's expected outage probability for a particularchannel is excessive, there may be a reasonable likelihood that theentire transmission on that channel will be corrupted and would need tobe re-transmitted. Furthermore, assignment of the channel may increasethe likelihood that transmissions by terminals in adjacent cells arealso corrupted by the additional interference. In such instances,assignment of the channel to the terminal may be unwise, and it may bebetter to not assign the channel at all or to assign the channel toanother terminal that may make better use of it.

The available channels may also be assigned to terminals with zero ormore conditions or constraints on usage. Such conditions may include,for example (1) limitation on the data rate, (2) maximum transmit power,(3) restriction on the setpoint, and so on.

A maximum data rate may be imposed on a channel assigned to an activeterminal. For example, if the expected C/I is not able to support therequired data rate, then the data rate may be reduced to achieve therequirement.

Maximum transmit power constraints may be placed on certain assignedchannels. If the cells in the system have knowledge of the powerconstraints for the channels in other cells, then the interferencelevels may be computed locally with higher degree of certainty andbetter planning and scheduling may be possible.

A particular setpoint may be imposed on an assigned channel, forexample, in heavily loaded situations. A (e.g., low priority) terminalmay be assigned a channel that does not meet the required minimum outageprobability (i.e., the assigned channel has an expected C/I that islower than required). In this case, the terminal may be required tooperate using the assigned channel at a lower setpoint that satisfiesthe required performance criteria. The setpoint employed may be staticor adjustable with system loading. Also, the setpoint may be imposed ona per channel basis.

Control Schemes

The adaptive reuse schemes, the scheduling of terminals for datatransmission, and the assignment of channels may be implemented invarious manners and using numerous control schemes such as centralized,distributed, and hybrid control schemes. Some of these control schemesare described below.

In a centralized control scheme, information from the active terminalsin all cells to be commonly controlled is provided to a centralprocessor that processes the information, schedules data transmissions,and assigns channels based on the received information and a set ofsystem goals. In a distributed control scheme, information from theactive terminals in each cell is provided to a cell processor thatprocesses the information, schedules data transmissions, and assignschannel for that cell based on the information received from theterminals in that cell and possibly other information received fromother cells.

A distributed control scheme performs scheduling of terminals for datatransmission and channel assignment at the local level. The distributedcontrol scheme may be implemented at each cell and involved coordinationbetween cells is not required.

In the distributed control scheme, local information may be shareddynamically with other cells in the system even though the schedulingand channel assignment may be performed locally at each cell. The sharedinformation may include, for example, the loading at a particular cell,a list of active terminals at the cell, channel availabilityinformation, the assigned back-off factors, and so on. In thedistributed control scheme, this information need not be shared in adynamic manner and may be “static” information available to the cells inthe system. The shared information can be used by the cells to helpdecide how to best allocate resources locally.

The distributed control scheme may be advantageously used under both lowand high load conditions, and is simpler to implement than thecentralized control scheme. At low load, the terminals in the cells aremore likely to be able to transmit using orthogonal channels, whichresults in minimal interference to terminals in other cells. As the loadincreases, the interference levels in the system will generally increaseand there is a higher likelihood that the terminals will be assigned tonon-orthogonal channels. However, as the load increases, the group ofterminals the cell can select from for scheduling also increases. Someof these terminals may be more tolerant of other-cell interference thanothers. A distributed control scheme exploits this fact schedulingterminals and assigning channels.

Distributed, centralized, and hybrid scheduling schemes are described infurther detail in U.S. Pat. No. 5,923,650, entitled “METHOD ANDAPPARATUS FOR REVERSE LINK RATE SCHEDULING,” issued Jul. 13, 1999, U.S.Pat. No. 5,914,950, also entitled “METHOD AND APPARATUS FOR REVERSE LINKRATE SCHEDULING,” issued Jun. 22, 1999, and U.S. patent application Ser.No. 08/798,951, entitled “METHOD AND APPARATUS FOR FORWARD LINK RATESCHEDULING,” filed Sep. 17, 1999, all assigned to the assignee of thepresent invention and incorporated herein by reference.

Power Control

Power control may be exercised by the cells for the assigned channels.If a terminal is assigned a channel and has positive link margin (i.e.,the difference between the expected C/I and the setpoint is positive),the transmit power of the terminal may be reduced based on thedetermined link margin. Even if other cells in the system are not awareof the reduced back-off for a particular channel, the overall effect isto reduce interference levels and improve the probability of successfultransmission. Power control may be performed dynamically, possibly insimilar manner as that performed for the uplink power control in CDMAsystems.

Combination with Other Reuse Structures

The adaptive reuse schemes described herein may also be implementedwithin or in combination with other reuse structures. One such structureis disclosed by T. K. Fong et al. in a paper entitled “Radio ResourceAllocation in Fixed Broadband Wireless Networks,” IEEE Transactions onCommunications, Vol. 46, No. 6, June 1998, which is incorporated hereinby reference. This reference describes partitioning each cell into anumber of sectors and transmitting to each sector at designated (andpossibly non-designated) and staggered time slots selected to reduce theamount of interference.

Another reuse structure is disclosed by K. K. Leung et al. in a paperentitled “Dynamic Allocation of Downlink and Uplink Resource forBroadband Services in Fixed Wireless Networks,” IEEE Journal on SelectedAreas in Communications, Vol. 17, No. 5, May 1999, which areincorporated herein by reference. This reference describes partitioningeach cell into a number of sectors and transmitting to each sector atdesignated (and possibly non-designated) and staggered time slots andsub-time slots selected to reduce the amount of interference. The C/I ofthe terminals are determined, and terminals are classified into groupsbased on their tolerance for up to q concurrent transmissions. Thetransmission pattern is then selected and data transmissions arescheduled to ensure conformance with the requirements of the terminals.

Yet another reuse structure is disclosed by K. C. Chawla et al. in apaper entitled “Quasi-Static Resource Allocation with InterferenceAvoidance for Fixed Wireless Systems,” IEEE Journal on Selected Areas inCommunications, Vol. 17, No. 3, March 1999, which is incorporated hereinby reference. This reference describes assigning each cell with a“beam-off” sequence and allowing the terminals to inform the cell thebest time slots for its data transmissions.

System Design

FIG. 9 is a block diagram of base station 104 and terminals 106 incommunication system 100, which is capable of implementing variousaspects and embodiments of the invention. At each scheduled terminal106, a data source 912 provides data (i.e., information bits) to atransmit (TX) data processor 914. TX data processor 914 encodes the datain accordance with a particular encoding scheme, interleaves (i.e.,reorders) the encoded data based on a particular interleaving scheme,and maps the interleaved bits into modulation symbols for one or morechannels assigned to the terminal for data transmission. The encodingincreases the reliability of the data transmission. The interleavingprovides time diversity for the coded bits, permits the data to betransmitted based on an average C/I for the assigned channels, combatsfading, and further removes correlation between coded bits used to formeach modulation symbol. The interleaving may further provide frequencydiversity if the coded bits are transmitted over multiple frequencysubchannels. In an aspect, the coding and symbol mapping may beperformed based on information provided by the base station.

The encoding, interleaving, and signal mapping may be achieved based onvarious schemes. Some such schemes are described in U.S. patentapplication Ser. No. 09/532,492, entitled “HIGH EFFICIENCY, HIGHPERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING MULTI-CARRIER MODULATION,”filed Mar. 22, 2000, U.S. patent application Ser. No. 09/826,481,entitled “METHOD AND APPARATUS FOR UTILIZING CHANNEL STATE INFORMATIONIN A WIRELESS COMMUNICATION SYSTEM,” filed Mar. 23, 2001, and U.S.patent application Ser. No. 09/776,075, entitled “CODING SCHEME FOR AWIRELESS COMMUNICATION,” filed Feb. 1, 2001, all assigned to theassignee of the present application and incorporated herein byreference.

A TX MIMO processor 920 receives and demultiplexes the modulationsymbols from TX data processor 914 and provides a stream of modulationsymbols for each transmission channel (e.g., each transmit antenna), onemodulation symbol per time slot. TX MIMO processor 920 may furtherprecondition the modulation symbols for each assigned channel if fullchannel state information (CSI) is available (e.g., a channel responsematrix H). MIMO and full-CSI processing is described in theaforementioned U.S. patent application Ser. No. 09/532,492.

If OFDM is not employed, TX MIMO processor 920 provides a stream ofmodulation symbols for each antenna used for data transmission. And ifOFDM is employed, TX MIMO processor 920 provides a stream of modulationsymbol vectors for each antenna used for data transmission. And iffull-CSI processing is performed, TX MIMO processor 920 provides astream of preconditioned modulation symbols or preconditioned modulationsymbol vectors for each antenna used for data transmission. Each streamis then received and modulated by a respective modulator (MOD) 922 andtransmitted via an associated antenna 924.

At base station 104, a number of receive antennas 952 receive thesignals transmitted by the scheduled terminals, and each receive antennaprovides a received signal to a respective demodulator (DEMOD) 954. Eachdemodulator (or front-end unit) 954 performs processing complementary tothat performed at modulator 922. The modulation symbols from alldemodulators 954 are then provided to a receive (RX) MIMO/data processor956 and processed to recover one or more data streams transmitted forthe terminal. RX MIMO/data processor 956 performs processingcomplementary to that performed by TX data processor 914 and TX MIMOprocessor 920 and provides decoded data to a data sink 960. Theprocessing by base station 104 is described in further detail in theaforementioned U.S. patent application Ser. No. 09/776,075.

RX MIMO/data processor 956 further estimates the link conditions for theactive terminals. For example, RX MIMO/data processor 956 may estimatethe path loss for each active terminal, the interference on eachchannel, and so on, which comprise channel state information (CSI). ThisCSI may be used to develop and adapt the reuse plan and to scheduleactive terminals and assign channels. Methods for estimating a singletransmission channel based on a pilot signal or a data transmission maybe found in a number of papers available in the art. One such channelestimation method is described by F. Ling in a paper entitled “OptimalReception, Performance Bound, and Cutoff-Rate Analysis ofReferences-Assisted Coherent CDMA Communications with Applications,”IEEE Transaction On Communication, October 1999.

A cell processor 964 at base station 104 uses the CSI to perform anumber of functions including (1) developing and adapting a reuse plan,(2) scheduling the best set of terminals for data transmission, (3)assigning channels to the scheduled terminals, and (4) determining thedata rate and possibly the coding and modulation scheme to be used foreach assigned channel. Cell processor 964 may schedule terminals toachieve high throughput or based on some other performance criteria ormetrics, as described above. For each scheduling interval, cellprocessor 964 provides a list of terminals scheduled to transmit on theuplink and their assigned channels and (possibly) data rates (i.e.,scheduling information). In FIG. 9, cell processor 964 is shown as beingimplemented within base station 104. In other implementation, thefunctions performed by cell processor 964 may be implemented within someother element of communication system 100 (e.g., a central controllerlocated in a base station controller that couples to and interacts witha number of base stations).

A TX data processor 962 then receives and processes the schedulinginformation, and provides processed data to one or more modulators 954.Modulator(s) 954 further condition the processed data and transmit thescheduling information back to terminals 106 via a downlink channel. Thescheduling information may be sent to the scheduled terminals by thebase station using various signaling techniques, as described in theaforementioned U.S. patent application Ser. No. 09/826,481, For example,the scheduling information may be sent on a designated downlink channel(e.g., a control channel, paging channel, or some other type ofchannel). Since the active terminals request the base station for datatransmission on the uplink, these terminals would know to monitor thedesignated downlink channel for their schedules, which would identifythe times they are scheduled to transmit and their assigned channels and(possibly) data rates.

At terminal 106, the transmitted feedback signal is received by antennas924, demodulated by demodulators 922, and provided to a RX data/MIMOprocessor 932. RX data/MIMO processor 932 performs processingcomplementary to that performed by TX data processor 962 and recovers aschedule, which is then used to direct the processing and transmissionof data by the terminal. The schedule determines when and on whichchannel the terminal is allowed to transmit on the uplink, and typicallyfurther identifies the data rate and/or coding and modulation scheme tobe used for the data transmission. If the terminal is not provided withinformation regarding which data rates to use on which channel, then theterminal may use “blind” rate selection and determine the coding andmodulation scheme. In this case, the base station may perform blind ratedetection to recover the data transmitted by the terminal.

The elements of the base station and terminals may be implemented withone or more digital signal processors (DSP), application specificintegrated circuits (ASIC), processors, microprocessors, controllers,microcontrollers, field programmable gate arrays (FPGA), programmablelogic devices, other electronic units, or any combination thereof. Someof the functions and processing described herein may also be implementedwith software executed on a processor.

Certain aspects of the invention may be implemented with a combinationof software and hardware. For example, the processing to schedule (i.e.,select terminals and assign transmit antennas) may be performed based onprogram codes executed on a processor (e.g., cell processor 964 in FIG.9).

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method for operating an uplink of a wireless communication system,comprising: partitioning available system resources into a plurality ofchannels; defining a reuse pattern for the communication system, whereinthe reuse pattern includes a plurality of cells; determining one or morecharacteristics for each cell in the communication system; allocating aset of channels to each cell based at least in part on the determinedone or more characteristics for the cell, wherein each allocated channelmay be assigned to a terminal for data transmission on the uplink; andrepeating the determining and allocating to reflect changes in thecommunication system.
 2. The method of claim 1, wherein each cell in thereuse pattern is allocated a respective set of channels that includesone or more channels available for transmission at full power level andone or more channels available for transmission at reduced power levels.3. The method of claim 1, wherein the set of channels allocated to eachcell is determined based in part on estimated loading conditions in thecell.
 4. A method for operating an uplink in a communication system,comprising: defining a reuse scheme to be used for data transmission bya plurality of terminals, wherein the defined reuse scheme identifies aparticular reuse pattern, an initial allocation of available systemresources, and a set of operating parameters; scheduling terminals fordata transmission in accordance with the defined reuse scheme; receivingtransmission from scheduled terminals; evaluating performance of thecommunication system; determining whether the evaluated systemperformance is within particular thresholds; and if the evaluated systemperformance is not within the particular thresholds, redefining thereuse scheme.
 5. The method of claim 4, wherein the defining the reusescheme includes developing characterization of interference received ateach cell in the communication system, partitioning the available systemresources into a plurality of channels, and allocating a set of channelsto each cell based at least in part on the developed interferencecharacterization for the cell.
 6. The method of claim 5, wherein thedefining the reuse scheme further includes defining a set of back-offfactors to be associated with each allocated set of channels.
 7. A basestation in a communication system, comprising: a resource allocationprocessor configured to receive data defining a reuse plan to be usedfor uplink data transmissions by a plurality of terminals, wherein thedefined reuse plan identifies a particular reuse pattern, an allocationof available system resources to a cell covered by the base station, anda set of operating parameters, wherein the resource allocation processoris further configured to schedule one or more terminals for datatransmission and to assign a channel to each scheduled terminal; atleast one front-end processor configured to process one or more receivedsignals from the one or more scheduled terminals to provide one or morereceived symbol streams; and at least one receive processor configuredto process the one or more received symbol streams to provide one ormore decoded data streams and to estimate one or more characteristicsfor the cell, wherein the resource allocation processor is furtherconfigured to receive channel state information (CSI) indicative of theone or more characteristics and to schedule terminals and assignchannels based on the CSI.
 8. The base station of claim 7, wherein theallocated system resources comprise a plurality of channels, and whereinthe resource allocation processor is further configured to determine aplurality of back-off factors for the plurality of channels based atleast in part on the CSI.